threads: add sysconf based number of CPUs detection

Can act as fallback and should work on a couple of Unix systems.

libavcodec/pthread.c

@@ -45,6 +45,9 @@
 #include <sys/types.h>
 #include <sys/sysctl.h>
 #endif
+#if HAVE_SYSCONF
+#include <unistd.h>
+#endif
 
 #include "avcodec.h"
 #include "internal.h"
@@ -177,6 +180,10 @@ static int get_logical_cpus(AVCodecContext *avctx)
     ret = sysctl(mib, 2, &nb_cpus, &len, NULL, 0);
     if (ret == -1)
         nb_cpus = 0;
+#elif HAVE_SYSCONF && defined(_SC_NPROC_ONLN)
+    nb_cpus = sysconf(_SC_NPROC_ONLN);
+#elif HAVE_SYSCONF && defined(_SC_NPROCESSORS_ONLN)
+    nb_cpus = sysconf(_SC_NPROCESSORS_ONLN);
 #endif
     av_log(avctx, AV_LOG_DEBUG, "detected %d logical cores\n", nb_cpus);
     return FFMIN(nb_cpus, MAX_AUTO_THREADS);